YM336D Wet Stacking?

[dumbfarmboy]

I'm interested to know what you find out, dumbfarmboy. Not to insult your intelligence, but when you rebuilt it did you hone the cylinders and then go through the break-in procedure? Without that, rings won't seat well, cylinders can glaze, and then comes low compression... Part of break-in is to run it hard, so that's a good thing to do even after the fact.

New sleeves, pistons, rings, etc. Sleeves looked like the horn job was right, all Yanmar parts. However I probably didn't run it hard enough during break-in. I'm not sure how it can start and run reasonably well if the compression test only shows a little over 200 psi. Maybe it doesn't run as good as I think it does or my compression gauge and/or adapter is faulty.

 

[rScotty]

New sleeves, pistons, rings, etc. Sleeves looked like the horn job was right, all Yanmar parts. However I probably didn't run it hard enough during break-in. I'm not sure how it can start and run reasonably well if the compression test only shows a little over 200 psi. Maybe it doesn't run as good as I think it does or my compression gauge and/or adapter is faulty.

If you look back at that chart I posted, an engine can start and run well but show low compression on a test if the starter is dragging - that is if it doesn't crank fast. If the compression gauge & adapter have a slight leak, that will have more of an effect if the starter is turning slowly.
Also it helps to have a new air filter - or remove the air filter element entirely - when doing a compression check.

New rings and a new engine will generally break in OK regardless of how it is run -especially with new cylinder liners. It doesn't take long to break in an engine. I'm not convinced that running them hard makes a positive difference. Race motors are heated up once, cooled down, checked over, and then ready to go.
rScotty

 

[Jerry/MT]

Ok, so a little more history. From fall 2019 until fall 2021 it sat and wasn't ran because I moved and left it with neighbors until I could go get it. Yesterday I fab'd a mock injector body and was able to run compression tests. Cold all within 5lbs of 200psi. Warmed up wasn't much different up to 210 PSI. Oiled cylinders and all came up to 265-275PSI. Service manual specs 568-639 PSI with a CR of 21:1 which calculates to only 308 PSI. I checked valve adjustment and all were correct. One thing I noticed was the casting # on the head is 3T90T-J which is the model # of the JD 1050 engine that's turbocharged. I was told it would work on the 3T84-N but I'm wondering if the pistons or head lower the compression for the turbo engine. Assuming it's like gasoline engines and run lower compression for turbos. Also when using MMO do I put it in the cylinders and if so, how much?

First of all, you are calculating the compression pressure from the compression ratio (its a volume ratio) and multiplying that by ambient pressure. That's incorrect.
Here's something I put together on the subject.

Engine compression ratio is actually a volume ratio not a pressure ratio. The equation I use to calculate the compression pressure is based on the isentropic equation pV^gamma=Constant (eqn 1) where p is the pressure, V is the volume and gamma is the ratio of specific heats of air.

Sparing you all the gory details, the equation for what you see on a compression gauge;

delta p = p1 x [((compression ratio)^gamma) -1], (eqn 2)

where p1 is atmospheric pressure, gamma is 1.4 and delta p is the compression gauge pressure

Now the isentropic equation assumes constant mass in the cylinder is constant but because of valve timing the mass in the real engine is not constant. So K is a constant that adjusts the theoretical value of compression gauge pressure to account for this.

Eqn 2 then becomes delta p = K x p1 x [(compression ratio)^gamma -1] (eqn 3)

Gamma =1.4. From experience K is between 0.70 and 0.75.
So a compression ratio 6.0 engine would have compression gauge pressure at an atmospheric pressure of 14.7 psia would be 124.6 psig (K=0.75). This is what you could expect as a maximum for a engine in good mechanical condition.

I hope this helps you.
SO a CR of 21
21^1.4 = 70.98

SO compression pressure = K X p1 x (69.98)
If p1 - 14.7 and K = 0.7 so the pressure should be around 720 psi ideally. This goes along with the 689 psig number you quoted which is most likey a new engine number and implies the K maybe lower than 0.7.
568 psig is most likely a lower limit that accounts for engine to engine variance. The 426 psig is a overhaul limit number. These are very high compression ratios as most NA diesels run at about CR 17.

 

[nyone]

I wouldnt condem the engine just yet. Get a proper compression gauge or pull the exhaust manifold first. Both cheap and can tell you more.